1. Field of the Invention
This invention relates to a process for the removal of a cyclic urea reaction product in an amine gas treating process which involves an absorption and a regeneration system. More particularly, the invention is concerned with the selective precipitation and filtration of a cyclic urea degradation product which forms as a by-product in the feed gas scrubbing process.
2. Description of the Prior Art
It is well known in the art to treat gases and liquids, such as mixtures containing acidic gases including CO.sub.2, H.sub.2 S, SO.sub.2, SO.sub.3, CS.sub.2, HCN, COS and oxygen and sulfur derivatives of C1-C4 hydrocarbons with amine solutions to remove these acidic gases. Th amine usually contacts the acidic gases and liquids as an aqueous solution containing the amine in an absorber tower with the aqueous amine solution contacting the acidic fluid countercurrently.
The acidic scrubbing processes known in the art can be generally classified into three (3) categories.
The first category is generally referred to as the aqueous amine process where relatively large amounts of amine solution are employed during the absorption. This type of process is often utilized in the manufacture of H.sub.2 for ammonia production where nearly complete removal of the acid gas, such as CO.sub.2 is required. It is also used in those instances where an acid gas, such as CO.sub.2, occurs with other acid gases or where the partial pressures of the CO.sub.2 and other gases are low.
The second category is generally referred to as the aqueous base scrubbing process or "hot potash" process. In this type of process a small level of an amine is included as an activator for the aqueous base used in the scrubbing solution. This type of process is generally used where bulk removal of an acid gas, such as CO.sub.2, is desired. This process also applies to situations where the CO.sub.2 and feed gas pressures are high. In such processes, useful results are achieved using aqueous potassium carbonate solutions as amine activators.
A third category is generally referred to as the non-aqueous solvents process. In this process, water is a minor constituent of the scrubbing solution and the amine is dissolved in the liquid phase containing the solvent. In this process up to 50% of amine is dissolved in the liquid phase. This type of process is utilized for specialized applications where the partial pressure of CO.sub.2 is extremely high and/or where many acid gases are present, e.g., COS, CH.sub.3 SH, and CS.sub.2.
The present invention relates to a process for the selective separation of a cyclic urea degradation product which may form as a by-product of the practice of the second category of acid scrubbing process described above, namely, the aqueous base scrubbing process or "hot potash" process in which a hindered amine is used.
Many industrial processes for removal of acid gases, such as CO.sub.2, use regenerable aqueous alkali scrubbing solutions, such as an amine and potassium carbonate which are continuously circulated between an absorption zone where acid gases are absorbed and a regeneration zone where they are desorbed, usually by steam-stripping. The capital cost of these acid scrubbing processes is generally controlled by the size of the absorption and regeneration towers, the size of the reboilers for generating stripping steam, and the size of the condensers, which condense spent stripping steam so that condensate may be returned to the system to maintain proper water balance. The cost of operating such scrubbing plants is generally related to the amount of heat required for the removal of a given amount of acid gas, e.g., thermal efficiency, sometimes expressed as cubic feet of acid gas removed per pound of steam consumed. Means for reducing the costs in operating these industrial processes have focused on the use of absorbing systems or combinations of chemical absorbants which will operate more efficiently and effectively in acid gas scrubbing processes using existing equipment.
It is disclosed in U.S. Pat. Nos. 4,112,050; 4,112,051 and 4,112,052 that sterically hindered amines unexpectedly improve the efficiency, effectiveness and cyclic working capacity of the acid gas scrubbing processes in all three of the above-mentioned process categories. In the case of the sterically hindered amine activated "hot potash" CO.sub.2 containing acid gas scrubbing process of the invention described in U.S. Pat. No. 4,112,050, the process can be operated at a cyclic working capacity significantly greater than when diethanolamine or 1,6-hexanediamine is the amine activator used in a similar process. It is postulated that the increase in cyclic capacity observed with the sterically hindered amines is due to the instability of their carbamates. In that respect, sterically hindered amines are similar to tertiary amines. Tertiary amines are not used on a commercial scale for carbon dioxide containing acid gas scrubbing due to their low rates of absorption and desorption.
N-alkyl alkylene diamines are advantageously used as sterically hindered amine activators in the "hot pot" process. A preferred sterically hindered amine used as an activator in the "hot pot" process is N-cyclohexyl-1,3-propanediamine. This amine in the presence of an amino acid is sufficiently water soluble under absorption and desorption conditions to maintain a single phase and it also has a very high absorption capacity.
Although N-cyclohexyl-1,3-propane diamine has been found to produce excellent results as an activator in the "hot pot" treating process, one drawback in processes where it has been used is that it produces a cyclic urea product when the acid treated gas is rich with CO.sub.2 and also contains H.sub.2 S. The cyclic urea has a deleterious effect on CO.sub.2 removal rates and must be removed and replaced with fresh N-cyclohexyl-1,3-propanediamine. The makeup rate for the hindered amine has a minimal effect on the process economics; however, the cyclic urea that is formed must be selectively removed in order to be able to maintain acid gas removal performance.
The invention which is disclosed herein represents an improvement to the "hot pot" amine activated gas treating process which includes the use of a hindered amine having a tendency to form cyclic ureas under CO.sub.2 rich conditions in the presence of H.sub.2 S. This invention discloses a processing step wherein the cyclic urea can be selectively removed from the circulating solution thereby preventing any loss in acid gas removal capabilities.